1. Field of the Invention
The present invention relates to reducing the non-volatile inorganic contaminant content of refinery waste water discharged to the environment. More particularly it relates to a process whereby selenium is removed from the waste water and concentrated and the selenium concentrate is disposed of in an environmentally responsible manner.
2. Description of the Prior Art
Selenium in waste water streams is recognized as a biological and health hazard and limits have been imposed on the amount of selenium which may be discharged into receiving bodies of water.
Because selenium is a component of refinery waste water, being a natural constituent of the crude oil refined, the restriction on the amount of selenium which may be discharged into receiving bodies of water necessarily acts as a restriction on how such refinery waste water streams can be handled and where and how they can be disposed and can influence the entire refining process.
Various techniques have been considered as a means for removing selenium from waste water streams.
Selenium can be removed from waste water streams, such as those associated with power plants, by iron coprecipitation. In that process a ferric iron salt is added to the waste water and a precipitate of ferric hydroxide is produced through the adjustment of the pH. The ferric hydroxide comes together as a flock like precipitate which adsorbs contaminants in the waste water on its surface. By careful control of the pH the selenium can be removed from the waste water. The treated waste water is sent to a settler and separated into clean water and a sludge containing the contaminants. Due to its high contaminants content (selenium and other metals etc.) the sludge is a hazardous waste and its disposal is regulated. Further, a very large volume of sludge is produced for the amount of contaminants removed.
Selenium can also be removed from waste water by contacting with elemental iron at low pH which converts the selenium compounds in the waste water into elemental selenium which is retained in the elemental iron. See U.S. Pat. No. 4,405,464 and U.S. Pat. No. 4,940,549. This process also produces a selenium containing sludge which poses disposal problems.
While evaporation is commonly used to concentrate inorganics in water streams, an issue exists as to what to do with the inorganics concentrate. Because refinery waste water streams contain many varied inorganic materials, evaporation practiced to concentrate the selenium would produce a concentrate stream containing large amounts of co-concentrated inorganics which would, again, constitute a disposal and handling problem.
It would be an advantage if selenium and other harmful, environmentally objectionable non-volatile inorganic materials could be removed from refinery waste water streams, so as to minimize sludge and contaminant concentrate handling problems and be disposed of in an environmentally responsible way.